Ternary Chalcogenides BaMxTe2(M = Cu, Ag): Syntheses, Modulated Crystal Structures, Optical Properties, and Electronic Calculations

Standard solid-state methods produced black crystals of the compounds BaCu0.43(3)Te2and BaAg0.77(1)Te2at 1173 K; the crystal structures of each were established using single-crystal X-ray diffraction data. Both crystal structures are modulated. The compound BaCu0.43(3)Te2crystallizes in the monoclinic superspace group P2(αβ1/2)0, having cell dimensions of a= 4.6406(5) Å, b= 4.6596(5) Å, c= 10.362(1) Å, β = 90.000(9)°, and Z= 2 and an incommensurate vector of q= 0.3499(6)b* + 0.5c*. The compound BaAg0.77(1)Te2crystallizes in the orthorhombic P21212(α00)000 superspace group with cell dimensions of a= 4.6734(1) Å, b= 4.6468(1) Å, c= 11.1376(3) Å, and Z= 2 and an incommensurate vector of q= 0.364(2)a*. The asymmetric unit of the BaCu0.43(3)Te2structure comprises eight crystallographically independent sites; that for BaAg0.77(1)Te2comprises four. In these two structures, each of the M (M = Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [MxTe4/4]n−that are separated by layers of Ba atoms and square nets of Te. A Raman spectroscopic study at 298(2) K on a pelletized polycrystalline sample of BaAg0.8Te2shows the presence of Ag–Te (83, 116, and 139 cm–1) and Ba–Te vibrations (667 and 732 cm–1). A UV–vis–NIR spectroscopic study on a powdered sample of BaAg0.8Te2shows the semiconducting nature of the compound with a direct band gap of 1.0(2) eV, consistent with its black color. DFT calculations give a pseudo bandgap with a weak value of the DOS at the Fermi level.